Chemical Feed, Storage, and Handling
Dose math, feed equipment, and chemical hazards
Chemical questions reward a few reliable formulas. Memorize lb/day = mg/L × MGD × 8.34 and its rearrangements; solution feed requires dividing by the active fraction and by the solution density (lb/gal = 8.34 × SG). Know metering pumps (diaphragm and piston), rotameters, calibration by drawdown of a graduated cylinder or tank sight glass, and independent verification of dose by jar test, residual, or alkalinity.
Chemicals: dry alum and ferric salts, lime (hydrated vs quicklime, with slaking heat hazards), soda ash, caustic soda, fluoride (about 0.7 mg/L optimal - watch saturation), potassium permanganate, and polymers (made down gently; shear destroys them). For disinfection know sodium hypochlorite strength and decay (heat and sunlight degrade it; trade strength near 12.5%; never mix with acid - chlorine gas is released), calcium hypochlorite (oxidizer, never with organics), and chlorine gas (about 2.5 times heavier than air, corrosive when wet, in 150-lb and 1-ton containers).
Storage rules: secondary containment, separation of oxidizers from acids and organics, eye wash and safety shower, correct PPE, dust control for lime, SDS access, and the correct A/B/C chlorine leak kit for the container.
Worked example: target 2.0 mg/L at 1.5 MGD using 12.5% hypochlorite (SG 1.17). Available chlorine = 2.0 × 1.5 × 8.34 = 25 lb/day. Solution = 25 ÷ 0.125 = 200 lb/day. Volume = 200 ÷ (8.34 × 1.17) ≈ 20.5 gal/day.
Traps: forgetting to convert solution strength; assuming a pump stroke setting equals delivered dose; mixing hypochlorite with acid.